Ice Nucleation in Cirrus Clouds: Anthropogenic Climate Forcing and Uncertainties

本研究的重点是利用全球化学/气候模式来改进卷云结冰的建模。这项研究将更好地使气候建模者了解对卷云中小颗粒（气溶胶）影响的模式预测差异的原因。它还将提供基于物理和化学过程的气溶胶和冰形成的描述，并与用于评估该模式的实地观测大体一致。研究人员假设，卷云中相当一部分冰的形成与次生有机气溶胶（SOA）和/或固体硫酸铵（NH4）2SO4有关，这些气溶胶随时间的变化导致了显著的气候强迫。将使用社区大气模式（CAM5）气候模式与IMPACT气溶胶模式相结合来检验这一假设。IMPACT模型将SOA的形成与基于最近扩展的氧化方案的挥发性有机化合物（VOCs）的光化学氧化联系起来。研究人员正在扩展SOA形成框架，以使其能够考虑形成机制如何改变与SOA相关的主要有机物和其他气溶胶物种的大小分布。在这项研究中，他们将探索几种参数化SOA成核的方法。他们将使用不同数量的灰尘、硫酸盐、玻璃状有机气溶胶（取决于每种SOA的玻璃化转变温度）和固体（NH4）2SO4来比较云中SOA形成和强迫的变化。模型模拟将与最近测量活动的观测结果进行比较，以确定SOA形成、冰云成核和上升气流速度的哪种处理方法最能再现观测结果。这项研究将使气候学界更好地了解卷云中气溶胶效应的模式预测差异的原因。这项研究的结果将有助于量化气溶胶对气候的影响，并帮助决策者了解气溶胶通过形成冰晶在加热或冷却气候方面可能发挥的作用。

This research focuses on the use of a global chemistry/climate model to improve the modeling of ice formation in cirrus clouds. This research will better inform climate modelers about the causes of differences in model predictions of the effects of small particles (aerosol) in cirrus clouds. It will also provide a representation of aerosol and ice formation that is based on physical and chemical processes and broadly consistent with the field observations used to evaluate the model.The researchers hypothesize that a substantial fraction of ice formation in cirrus clouds is associated with secondary organic aerosols (SOA) and/or solid ammonium sulfate (NH4)2SO4 and that changes in these aerosols over time result in a significant climate forcing. The hypothesis will be tested using the Community Atmosphere Model (CAM5) climate model coupled to the IMPACT aerosol model. The IMPACT model ties the formation of SOA to the photochemical oxidation of volatile organic compounds (VOCs) based on a recently extended oxidation scheme. The researchers are in the process of extending the SOA formation framework to allow it to consider how the formation mechanism alters the size distribution of both primary organics and other aerosol species associated with SOA. In this research, they will explore several ways of parameterizing SOA nucleation. They will compare the changes in SOA formation and forcing in cirrus clouds using varying amounts of dust, sulfate, glassy organic aerosols (depending on the glass transition temperature of each SOA), and solid (NH4)2SO4. The model simulations will be compared with observations from recent measurement campaigns to identify which treatments of SOA formation, ice cloud nucleation, and updraft velocities are best able to reproduce the observations. This research will better inform the climate community of the causes of differences in model predictions of aerosol effects in cirrus clouds. The results of this research will aid in quantifying climate forcing by aerosols and help to inform policy makers of the possible role of aerosols in either heating or cooling climate through their action in nucleating ice crystals.